Impact of season of harvest on in vitro gas production and dry matter degradability of Acacia saligna leaves with inoculum from three ruminant species

In vitro gas production (IVGP) and dry matter degradability (IVDMD) of Acacia saligna leaves (ASL) from four seasons were studied under arid Egyptian conditions as a 4 × 3 factorial experiment (4 seasons × 3 ruminant species). Incubations were completed using rumen liquid collected immediately after slaughter from sheep, cattle and buffalo, in order to investigate differences among ruminants in their ASL fermentation capacity. Samples of ASL were collected during the last 2 months of each season, being autumn, winter, spring and summer (between the 5th and 12th week of each season). Dried samples of ASL were incubated for 24 h in each of the three buffered rumen liquors, using a syringe technique, to determine IVGP and IVDMD. The crude protein content of ASL was lower (P<0.01) in summer (143 g/kg DM) than autumn (171 g/kg DM), winter (177 g/kg DM) and spring (182 g/kg DM). In winter, ASL had lower neutral detergent fibre, acid detergent fibre, acid detergent lignin and cellulose than in other seasons (P<0.05), but there were no differences among seasons in ash and hemicellulose contents. Condensed tannin (CT, as quebracho tannin equivalent) contents of ASL were higher (P<0.001) in summer (113 g/kg DM) versus the other seasons, with the lowest value during winter (63 g/kg DM). Gas production after 24 h was higher (P<0.05) with buffalo rumen fluid, versus cattle or sheep, in all seasons except winter. IVGP with buffalo rumen fluid was not affected by season but, with cattle and sheep, IVGP was higher (P<0.01) in winter. IVDMD was higher in winter and spring, and lower in summer and autumn, within all species, and higher (P<0.001) values were general with buffalo versus other species. IVGP was positively (P<0.05) correlated with IVDMD, but there was no consistent relationship between IVGP or IVDMD and chemical composition of ASL. Rumen fluid from buffalo, cattle and sheep have different capacities to ferment A. saligna leaves, and differences among species were smallest in winter, when the fibre and CT contents of ASL were lower.